# DIFFICULTY: SIMPLE

#Assumptions: A Plaintext represents the original message. A Ciphertext represents the encrypted message. The keys are used to encrypt the plaintext. Consider \{A = 0, B = 1, \cdots ,Z = 25\} . There are no spaces, newline or other punctuation symbols in plain text

#Key Generation : Alice chooses two keys K_1, K_2 < 26 and securely send K_1, K_2 to Bob.

#Encryption : Encryption function is C = (P \times K_2) + K_1 \ mod \ 26 ; where P is the plaintext and C is the ciphertext.

#Decryption : Decryption function is P = (C − K_1) \times K_{2^{−1}} \ mod \ 26 ; where P is the plaintext and C is the cipher text; K_{2^{−1}} is the multiplicative inverse in GF(26) i.e. 1 = K_{2^{-1}} \times K_2 \ mod \ 26 .

#Example: If the Plaintext: HELLOWORLD = {7 4 11 11 14 22 14 17 11 3} Keys : K_1 = 3, K_2 = 7. K_{2^{-1}} = 15 as 7 \times 15 = 1 \ mod \ 26 . So the Cipher Text: AFCCXBXSCY = {0 5 2 2 23 1 23 18 2 24}.